Fluorine-containing cation exchange membranes are widely used or proposed to be used as ion exchange membranes for electrolysis to produce an alkali metal hydroxide and chlorine or as diaphragms for water electrolysis, hydrochloric acid electrolysis or electrolysis for recovery of valuable metals, by virtue of their excellent heat resistance, chemical resistance and mechanical resistance. When a fluorine-containing ion exchange membrane is practically used for electrolysis, it is common to incorporate into the membrane a porous base material such as a woven fabric made of a fluorine-containing polymer such as polytetrafluoroethylene (PTFE) as a reinforcing material and thereby to support the membrane (e.g. Japanese Unexamined Patent Publications No. 56192/1978, No. 37186/1983 and No. 37187/1983).
However, with such a reinforced fluorine-containing ion exchange membrane, the fluorine-containing polymer such as PTFE used as the reinforcing material, is likely to shield permeation of ions (flow of electric current), whereby the membrane electric resistance tends to increase.
It is effective to reduce the thickness of the fluorine-containing polymer having ion exchange groups for the purpose of reducing the membrane resistance. However, such an attempt usually leads to a problem that the strength, particularly the bending strength, deteriorates due to the notch effect derived from the contour of yarns constituting the woven fabric. Namely, the resistance against bending corresponds to the cube of the film thickness. Therefore, an ion exchange membrane made of a thin film tends to be inadequate, particularly in the bending strength. There has been no practically useful low electric resistance membrane having a small thickness. Fluorine-containing polymer membranes having ion exchange groups and reinforced by woven fabrics, which are commercially available for practical use, have a thickness of at least 150 .mu.m, as measured by a weight method.